1. Field of the Invention
This invention relates to a distal centralizer for the stem of an intramedullary prosthesis such as a femoral implant. More particularly, it relates to a distal centralizer having four elements extending outwardly therefrom in a direction generally perpendicular to the stem with neighboring fins being offset from one another in the proximal distal direction.
2. Description of the Prior Art
Distal centralizers are used to locate the stem of a cementable prosthetic orthopedic implant in a neutral position within the medullary canal of a bone. This allows for the formation of a uniform bone cement mantle between the implant and the bone.
In general, prior art centralizers have been formed as tapered rings with fins around their periphery extending radially outwardly therefrom. The centralizer is placed over the tip of the stem and slid along a tapered portion of the stem. Usually fixation to the stem is accomplished by forming a matching taper on the inner surface of the centralizer corresponding to the taper on the stem.
Centralizers for example, as shown in British Patent Application 2 104 391 are used to centralize the distal stem in the femoral canal. Another centralizing device using vertically extending elements is shown in U.S. Pat. No. 3,793,650, which issued to the inventors of U.S. patent application 2,104,391. A polyethylene sleeve is shown in the Exeter Universal Hip system and is similar to that of 3,793,650 in that the spring elements extend vertically. It will be appreciated that these spacers are not only intended to centralize the distal tip of the prosthesis, but also to prevent the stem from actually engaging the wall of the canal.
A disadvantage with some of the prior art devices is that, when used in cemented applications, they tend to accumulate voids and air bubbles in the cement behind them as they are pushed downwardly; these voids and air bubbles form behind the fins, that is, between the fins and the metal stem of the prosthesis. This is to be avoided as it can become a point of weakness and crack initiation within the cement mantle.
Polymethylmethacrylate sleeves are known and have been taught by Leo A. Whiteside et al. in an article published in June 1988 entitled "The Effects of the Collar on Total Hip Femoral Component Subsidence" and sold as the Whiteside Total Hip System. However, this device is merely a cylindrical sleeve which can only centralize the distal stem in a precisely reamed canal.
The design of the present invention has four fins equally spaced around the periphery of the ring body of the centralizer. This enhances the ability of the centralizer to centralize in a non-circular intramedullary canal. While a greater number of fins could further increase the centralization potential of the centralizer, such a design can choke off the flow of cement around the fins upon insertion of the stem with the centralizer mounted thereon into the canal.
In addition to the fins being equally spaced, opposing pairs of the four fins are offset from one another in the proximal-distal (i.e., longitudinal) direction. This offsetting of the fins promotes in the spreading out of any discontinuities in the fresh cement beyond a single plane. The probability of crack propagation across the cement mantle in one plane is thereby reduced. It has also been found that the angle between the long axis of the stem and the leading edge of each of the attached centralizer fins should be 135.degree. or greater, which reduces the number of cement defects caused by insertion of the component due to optimized flow dynamics. It has likewise been found that the trailing edge of each fin should be less than 90.degree. to the long axis of the femoral component to reduce defects in the mantle. As a result of the fin pairs being offset from one another and the ring being of constant length, the tapered ring takes on a wavelike form. Thus, if sectioned longitudinally, the proximal-distal dimension of the ring remains constant giving the wavelike structure. It has been found that this distance between distal and proximal ends of the centralizer should be a maximum of 10 mm. Beyond 10 mm the rejoining of the cement flow over and beneath the ring is difficult to attain during insertion into the femoral canal. This is especially the case when the centralizer is used with a stem having grooves or depressions formed therein which allow the cement to flow both over and under the centralizer.